This present invention relates to a light-emitting element drive circuit and, more particularly, relates to a circuit for driving a light-emitting element for use with a camera.
Recently, there has been an increasing trend toward miniaturization of cameras, and it is now commonplace to use a single 3-volt lithium battery as the power source for driving the entire system of a camera. If necessary, the battery output voltage may be boosted by a booster circuit. For example, when a light-emitting diode is used as a light-emitting element in the known active-type automatic focusing circuit, it requires a suitable forward voltage, and a camera using a 3-volt battery is, in many cases, driven by a boosted power source.
One example of a conventional light-emitting diode drive circuit for a camera is shown in FIG. 3. As shown, a battery 30 supplies power to each of the circuits constituting the system of a camera. A coil 31, a transistor 32, a diode 33 and a capacitor 34 are connected as shown and constitute a known chopper-type booster circuit. The transistor 32 repeats an on/off operation at a high frequency (on the order of about several tens to several hundreds kHz) according to a signal from a booster control circuit 32a. When the transistor 32 is on, the potential of point Pa is substantially equal to ground potential so that the coil 31 generates a counter-electromotive force at both ends thereof and stores electric energy therein. On the other hand, when the transistor 32 is off, the electric energy stored in the coil 31 is outputted to the post-stage circuits through the diode 33, and a part of the energy is stored in the capacitor 34. The input and output energy levels of the capacitor 34 can be balanced by suitably modifying the duty ratio of the pulse signal for driving the transistor 32 so that the voltage at point Pb becomes a predetermined value. As a result, a constant voltage to be supplied to the circuit 35 which constitutes the system of the camera appears at point Pb.
A resistor 36 is a low level resistor connected to point Pb for supplying electrical charge to a capacitor 37. The capacitor 37 stores the electric charge supplied thereto through the resistor 36. A transistor 39 is connected in series with a near-infrared light-emitting element 38 to control current flow through the element 38 under control of a light-emitting signal generating circuit 39a. If a sufficient electrical charge is stored in the capacitor 37 and the transistor 39 is off, then the electric potential at a point Pc is substantially equal to that of point Pb. When a light-emitting signal is outputted from the light-emitting signal generating circuit 39a, the transistor 39 is turned on and drives the near-infrared light-emitting element 38, and the electrical charge stored in the capacitor 37 flows straight to the element 38 to cause the latter to emit light. When the transistor 39 is turned off to terminate the emission of light, the potential at point Pc is lower than the potential at point Pb and, therefore, the capacitor 37 is again supplied with electric charge through the resistor 36.
Further, there is described in Japanese Laid-Open Utility Model Publication No. 61-142463 a booster circuit which uses two switching elements Q1 and Q2 as shown in FIG. 4. This circuit includes a diode D1 as a light-emitting element. A capacitor C1 is charged through a charging circuit comprised of a power source Vcc, a resistor R5, a resistor R2, a capacitor C1, a resistor R4 and ground. The diode D1 is driven through a driving circuit comprised of the power source Vcc, the resistor R5, the transistor Q2, the diode D1, the capacitor C1, the transistor Q1 and ground.
The drive circuit shown in FIG. 3 has several disadvantages. Firstly, since the circuits constituting the camera system and the light-emitting element drive circuit are connected to the same system power source, the system power source voltage fluctuates due to the emission of near-infrared light by the light-emitting element 38 and the fluctuations become noise. In this case, if the light-emitting cycle of the near-infrared light-emitting element 38 is attempted to be shortened, the consumption of power per unit hour increases thereby increasing the possibility that the camera system will malfunction due to a lowering of the power source voltage. Moreover, since the capacitor 37 is generally required to be an electrolytic capacitor of large capacity, such is disadvantageous from the standpoint of miniaturization and space reduction of the camera.
Further, as the circuitry shown in FIG. 4 requires the provision of an independent drive circuit for driving the light-emitting element, it is also disadvantageous from the standpoint of miniaturization and space reduction.